Information from 1996 Missouri Rice Research Update.

Control of Algae in Rice Field Soils1

Steven Hefner, Ray Archuleta, Andy Kendig, and Gene Stevens2

Waterseeding rice culture is a viable alternative to Missouri
rice producers who continue to battle red rice infestations. This
production practice usually involves early flooding of zero slope
fields to suppress red rice weed germination. Fields are then
planted by broadcasting pre-sprouted rice seeds into the
floodwater. Although early flooding suppresses red rice, it can
promote algae formation. Algae proliferation is often detrimental
to rice stand establishment, especially in fields with high
inoculation levels.

Control methods for algae control are limited at best.
Draining fields is usually not an option because of loss of
agrichemicals in floodwater. Draining fields after algae mats have
formed can weigh emerging seedlings downward. The absence of
floodwater also promotes emergence of weed species (e.g.
sprangletop, barnyardgrass, red rice). Applications of granular
copper compounds have been used with mixed results. Successful
algae control with copper sulfate seems to be linked with early
scouting and application.

Although never tested for rice production, a double salt of
ammonium sulfate and aluminum sulfate (commonly called alum) has
properties that could inhibit algae growth. Alum is frequently used
in waste water facility plants to precipitate solids in water.
Recent research in Arkansas showed that alum applied to litter in
chicken houses reduced ammonia gas levels. Alum applied to chicken
manure also reduced phosphorus solubility and runoff into streams
when manure was applied to pastures.

A greenhouse study was initiated on March 11, 1996 at the
University of Missouri Delta Center, Portageville, MO. The
objective of the greenhouse study was to screen possible algae
control strategies. Treatments which show merit can be further
tested both in the greenhouse and in the field.

All copper sulfate and Earthtec* applications were applied
with a pressurized backpack sprayer. Earthtec* is a liquid copper
sulfate formulation. Alum (aluminum ammonium sulfate) applications
were broadcast preflood onto soil in tank 9 and into an algae bloom
in floodwater in tank 10. Tank 17 was acidified after flooding with
one sulfuric acid application. An arbitrary scale was used to
visually rate tanks for algae growth at least every other day
(Table 2). The pH level of floodwater was monitored at least every
other day with a Cardy Twin pH B-113 portable pH meter.

Rice (Oryza sativa L., cv. L202) seeds were subjected to both
a 24 hour wetting and subsequent 24 hour drying period prior
planting. Rice was planted on March 15, 1996 at a seeding rate of
52 seeds/tank. Warm temperatures and a shallow flood accelerated
seedling emergence. Plants were out of the water in 3 days. Stand
counts were made on March 21, 1996. Whole plants were collected,
oven dried (110 F for 2 days), and weighed for biomass measurement
comparisons.

Results

Greenhouse conditions were optimum for algae proliferation.
Daytime soil temperatures approached 95F and overnight low
temperatures were 80F. Within 30 hours of flooding, algae had
developed at least 50% of the soil surface in half of the tanks.
Tanks receiving either copper compounds or postflood N management
had less initial algae growth than the untreated check with
preflood urea. The algae colony was identified as a mixture of
Eucaryotic algae containing a filamentous Ulothrix species and two
flagellated species, Euglena and Chlamydomonas3.

Table 2. Visual rating scale for algae growth.

Rank Description
1 clean tank
2 25% of the soil surface covered with algae
3 50% of the soil surface covered with algae
4 66 % of soil surface covered by an algae
5 83% of soil surface covered by an algae
6 100% of soil surface covered by an algae
7 algae floating on water surface
8 33% of water surface covered by algae
9 33 - 66% of water surface covered by algae
10 100% of the water surface covered by algae

Copper sulfate dissolved into solution and Earthtec* performed
at the same level of effectiveness. Granular copper sulfate is
somewhat difficult to dissolve into solution. Preflood application
timings were lesseffective than postflood applications. Tanks
receiving both preflood applications of N and 5 lbs/a of copper
sulfate slowed algae growth from 3 to 5 days. Acidifying the
floodwater to an initial pH of 2.0 extended control of algae with
copper sulfate to 7 days.

Increasing the quantity of copper sulfate to 8 lbs/a and
applying into floodwater before the algae colony floated to the surface extended control to 7 days. The copper sulfate
applications were made two days prior to planting. These
applications at 8 lbs/a of copper sulfate equivalent decreased
rice stand counts for both Earthtec* (16 plants/tank) and copper
sulfate (6 plants/tank). Plants that did emerge were chlorotic
even though preplant N had been applied.

Nitrogen fertilizer stimulated algae growth. Tanks receiving
preflood urea or ammonium sulfate, but no copper containing
compounds experienced immediate algae pressure. Delaying the N
until rice seedlings emerged slowed algae growth. Subsequent
fertilization further stimulated algae proliferation. No
differences in algae growth were noticed between urea and ammonium
sulfate N sources. Knifing aqua ammonia into the soil was
superior to broadcasting either urea or ammonium sulfate on the
surface. The aqua ammonia tank had the most plants emerging (42) of
any in the test.

Both preflood and postflood applications of alum were included
in the test. The preflood application at 100 lbs ammonium alum was
not effective, but some inhibition of algae growth was observed.
Applying 1000 lbs/a postflood into an floating algae mat was highly
effective.

The postflood application of alum lowered the floodwater pH
from 8.2 to 4.2, The algae mat soon settled from the water surface
to the soil surface. It is unclear if the algae control experience
from this treatment was the result of the pH floodwater shift, the
physical sedimentation of the algae bloom, or possibly a
deprivation of available phosphorus brought about by the aluminum
in the alum.

Conclusions

Copper sulfate compounds were somewhat effective at
controlling algae growth under accelerated conditions. Soil
applied preflood applications were less effective than applications
into floodwater. Delaying N until rice emergence was also
effective at slowing algae growth. However, postflood applications
of urea are also terribly inefficient for a production basis. The
alum treatments show merit for further testing.

At the onset of the experiment, we were anxious about getting
the algae to grow in the greenhouse. Warm conditions were
established to enhance algae growth. A replicated test is now
underway in a cooler environment more representative of seasonal
May temperatures. This study includes copper sulfate sources,
quantities and timings. Floodwater pH levels and alum quantities
are also being examined. Field test are planned for the upcoming
season.